Recovery or removal of ethanol from aqueous solutions is needed in many industrial processes and in the clean-up of waste streams. Recently increased use of fermentation to produce ethanol also leads to recovery problem of this kind. Because of relatively great affinity between ethanol and water, significant energy is required to achieve separation using the conventional evaporation and distillation techniques. Many attempts therefore have been made to develop less energy-intensive separation alternatives which include adsorption, liquid extraction, reverse osmosis membrane processes, and supercritical fluid extraction.
Because supercritical carbon dioxide possesses several special characteristics and physicochemical properties, such as nonflammable, nontoxic, inexpensive, higher mass transfer rate, and adjustable extraction power for organic compounds varying with the density, it has been recognized to be a good solvent for extraction of organic compounds from aqueous solution. Despite of low distribution ratio for dilute aqueous solutions and phase complexity near the critical point of carbon dioxide, as well as several technical problems remain to be solved such as clean-up and recompression of recycled carbon dioxide, to extract ethanol from aqueous solutions by supercritical carbon dioxide seems to be superior to distillation technique from the energy point of view. McHugh and Krukonis in their book, entitled "Supercritical Fluid Extraction Principles and Practice", Butterworth, Stoneham, MA (1986), have extensively described the supercritical fluid extraction, details thereof are incorporated by reference.
Ethanol separation by reverse osmosis membrane process has been a subject over the past years. The reverse osmosis can be illustrated by its application in sea water purification. Imagine a box divided into two chambers by a rigid, semipermeable membrane that allows water to pass through it but does not allow the passage of solutes contained in sea water. In the left chamber we put pure water, and in the right, we put sea water. Since water activity in the left (pure water chamber) higher than that in the right (sea water chamber), water will flow through the membrane from left to right and dilute the sea water. This phenomenon is called osmosis. If the pressure of sea water chamber is increased gradually, the osmosis phenomenon will become insignificant. When the increase of pressure stops water flowing through the membrane from left to right, the extra pressure applied is called osmotic pressure. If the pressure applied in the right chamber (sea water) is higher than the osmotic pressure, water in the right (sea water) chamber will flow through the membrane to the left (pure water) chamber. As a result, water is recovered from the sea water chamber. This phenomenon is called reverse osmosis. In separating a solution by reverse osmosis, the solution to be separated flows on one side of a membrane, the solution collected on the other side is called permeate and the solution remained on the same side is called retentate. The separation effectiveness is generally determined by the rejection rate, R, of a subject solute, which is defined as follows, and the permeation rate of the solution EQU R=(C.sub.in -C.sub.perm)/C.sub.in .times.100% (I)
wherein
C.sub.in is the concentration of a subject solute in the feed solution; PA1 C.sub.perm is the concentration of the subject solute in the permeate.
According to the definition, a higher rejection rate represents a better separation effectiveness.
In most of the membrane separation process a plurality of membranes are used as a module. Typical modules including spiral type, tubular type, hollow fiber and frame/plate type. The reverse osmosis systems in general use the spiral type module. The membrane can be classified by its synthesizing method, for example a dense film, an asymmetric film, or a composite film. A dense film is suitable for using in a high pressure separating operation, which is homogeneous and is synthesized uniformly. An asymmetric film is synthesized under nonhomogenous conditions, each surface of which has a different density. The asymmetric film is used in a relatively low pressure reverse osmosis separation. A composite film is prepared by modifying a dense film or an asymmetric film or by blending several different polymeric films. A recently developed composite film is directly synthesized. The composite films in the prior art mostly are specially designed for separating certain particular solutions.
In the prior attempts for separating ethanol from an aqueous solution by reverse osmosis, it is found that the separation effectiveness is not satisfactory because water molecules in the membrane matrix tend to form dimers which have a molecular weight very close to that of ethanol.
To enhance separation effectiveness, one of the approaches is to synthesize new membranes or to moditfy the existing membranes, and the other approach is to develop new separation processes. Pervaporation and perstraction are two examples for the latter one in which a low-pressure vapor or a purge organic liquid is allowed to flow along one face of a membrane, while the feed flows along the opposite face. A better separation provided by these processes over the use of membrane alone may be due to the addition of a driving force for mass transfer. Particular references are as follows:
1) "Observations on the Performances of Pervaporation under Varied Conditions. In Membranes and Membrane Process" by Rautenbach and Albercht; Drioli, E., Nakagaki, M. Eds.; Plenum Co.: New York, 1986, p 595-607.
2) "Separation of Liquid Benzene/Cyclohexane Mixtures by Perstraction and Pervaporation" J. Membrane Sci. 1988, 77, 205-232, by Acharya et al.
3) "Sorption, Diffusion, and Pervaporation of Organics in Polymer Membranes" J. Membrane Sci. 1989, 44, 161-181, by Lee et al.
4) "On the Prediction of Separation Factor and Permeability in the Separation of Binary Mixtures by Pervaporation" J. Membrane Sci. 1989, 46, 335-348, by Rhim and Hwang.
5) "A Study on Characteristics and Enhancement of Pervaporation-Membrane Separation Process" Desalination 1989, 71, 1-18, by Zhu et al.
The object of present invention is to provide an improved process for ethanol separation from an aqueous solution by incorporating supercritical carbon monoxide with reverse osmosis.